Vehicle motion is highly dependent on the friction coefficient, i.e. the friction between the tires and ground. Therefore an accurate estimate of friction is valuable for many active safety functions such as collision avoidance. For example if low tire-to-road friction can be detected, the driver can be alerted, brake intervention can be performed earlier and the risk of collision and serious injuries can thereby be decreased. It is also expected that autonomous driving will require accurate friction estimation to adapt vehicle speed automatically, where driving fast on low friction surfaces could cause hazards such as skidding.
Since it is typically both difficult and costly to directly measure friction, the research society and car industry has been focusing on technologies where the tire itself acts as the friction “sensor”. That is, by utilizing vehicle dynamics models which relate e.g. tire slip and force as a function of friction, together with motion sensors such as inertial measurement units, wheel-speed sensors, etc., friction can be estimated.
US2005/0033499 discloses a method for estimating the road-to-tire friction in order for a collision avoidance system to adapt to current road friction conditions. In the method described in US2005/0033499, wheels are actively excited by applying an opposing torque to wheels on the respective first and second axle.
However, a major limitation of current slip-force based estimation technologies is that estimation of slip is very difficult to perform in the case where all wheels experience longitudinal wheel forces e.g. during braking and/or all-wheel drive. Moreover, many existing friction estimation algorithms are active only during aggressive maneuvers, e.g. during acceleration or braking or cornering. The problem is also to estimate friction coefficient during low force excitement of the tires.
Accordingly, there is a need for an improved system and method for determining tire-to-road friction in a vehicle.